This invention relates to press felts, which are used in the press section of a paper making machine. In this part of the papermaking machine, the paper web, which may contain up to about 85% water, is passed between pairs of opposed rolls so that a large proportion of this water is literally squeezed out of the wet paper web. In this process, the wet paper web is supported by at least one, or conveyed between two opposed fabrics known as press felts. These press felts are generally constructed with a fabric base structure, which has layers of batt needled to at least one, and generally to both, of its surfaces. Several fabric constructions are used for the base fabric structure, including both single layer and double layer woven fabrics, and the fabrics can be woven either flat as a continuous run, or endless as a closed loop. Endless felts require that the press section be opened up to allow their installation. Seamed felts are open ended for installation and the ends are subsequently joined by seaming, for example with a pin seam, on the press section. In either case, in order to be able to install the finished fabric into a press section, it is often advantageous to make provision for a seam that can be closed during installation in the press section.
The seam is the most difficult part of the press fabric to make. For although it is relatively straight forward to create a press felt which will provide the required properties of adequate strength, adequate drainage, and lack of fabric marking, whether or not it can be used depends upon also being able to provide a seam in the fabric which has an adequate life in service, provides the required strength, provides the required drainage, and most importantly does not cause marking of the paper.
The most commonly used seam in press felts is a pin seam, in which monofilament yarns from each of the two ends of the fabric which are to be joined are woven back into the fabric ends to provide a set of small loops. The two fabric ends are joined by interdigitating the two sets of loops, and inserting a pintle across the width of the fabric. This form of seam is most often used in press felts based on double layer woven fabrics. Examples of such seams are shown in EP 0 294 951, U.S. Pat. No. 4,842,925 and U.S. Pat. No. 4,601,785.
These conventional press felts offer the benefits of ease of installation, and ease of seam assembly on the machine. But they have the disadvantages of being limited to applications which can tolerate at least some marking of the paper by the seam, and where the drainage requirements are modest. A further disadvantage is that the applied batt wears more rapidly in the seam area. An area of worn batt cover over the seam area increases the danger of marking, and also accelerates even more the rate of wear in the seam area. This leads to premature press felt failure due to either paper marking problems, or mechanical seam failure, or both.
Several proposals have been made to overcome these difficulties.
One that has been made is to use two woven fabric layers, rather than one, to provide what may be termed a laminated base structure. Fabrics of this type are shown in U.S. Pat. No. 3,086,276, wherein the fabric layers are disposed in a bias relationship, and in U.S. Pat. No. 3,657,068 in which a relatively fine and a relatively coarse fabric are disposed on each side of a layer of batt. An alternative fabric is shown in U.S. Pat. No. 5,110,672. In these references, the layers are laminated together by the batt which in the needling application process is forced into engagement with all of the layers. A problem common to all of these structures is that the cut fabric ends adjacent the seam tend to unravel, thus leaving the batt more or less unsupported in the most important area of the fabric. This leads to marking of the paper, and to premature seam failure.
Alternatively, it has been proposed to use a laminate structure, examples of which are described in U.S. Pat. No. 4,186780 and in U.S. Pat. No. 4,418,726, and to pin seam each of the fabrics in the laminate. Apart from the fact that this does not eliminate any of the difficulties attendant on a single pin seam, it further complicates installation, as there are now two seams to be closed, not one. Such an installation step is difficult, time consuming and defeats the original purpose of the seam, which is to provide ease of installation.
Another proposal is to provide what may be termed a “flap” of batt, which may be reinforced with a portion of the fabric base, that will overlay a conventional pin seam, preferably on the paper side of the fabric. Examples of this are described in CA 1,303,833 and EP 0 294 951 mentioned above. In each of these, in constructing the pin seam, a short flap of batt is provided which is attached to one of the ends to be joined and which is intended to cover the joint after the pin seam has been assembled. In U.S. Pat. No. 4,601,785 it is recommended to attach the batt flap by gluing or needling. In U.S. Pat. No. 4,842,925 it is recommended to use brittle or soluble yarns adjacent the seam. Either during or following the needling step to attach the batt these yarns are removed thus allowing the batt flap to be cut and placed in position. In EP 0 294 951 it is recommended to use an additional layer of machine direction yarns in the base fabric to reinforce the batt flap. Seams such as these are also difficult to assemble, and suffer from the disadvantage that the fabric in the flap supporting the batt also ends to unravel, thus leaving the batt unsupported which results in marking of the paper.